The present invention relates to the general field of mass storage of energy by compressing air. It relates more particularly to controlling the temperature of a heat regenerator used in an installation for storing energy by adiabatic compression of air.
Storing energy by compressing air consists in storing compressed air for subsequent use as a source of mechanical energy. Typically, the installations that are used for this type of energy storage are subjected to successive operating cycles, each operating cycle comprising a compression stage during which the air is compressed to high pressure and stored in a storage cavity, and an expansion stage during which electricity is produced by expanding the compressed air through air turbines driving alternators.
The principle of storing energy by compressing air is to take advantage of price differences for electricity between so-called “off-peak” hours during which electricity rates are less expensive, and so-called “peak” hours during which rates are more expensive. The energy-consuming compression stages are advantageously performed at lower cost during off-peak hours, while the electricity-producing expansion stages are performed during peak hours in order to supply electricity that is injected into electricity transmission networks when rates are more advantageous.
Various types of installation exist for storing energy by compressing air. Some such installations are said to be “diabatic” since they do not recover the heat that results from compressing air during the compression stages. As a result, they present relatively low electrical efficiency (less than 50%). They are also polluting since they eject CO2 emissions that result from burning a fossil fuel, as is essential for preheating the air before it is sucked into the air turbines during the expansion stages. Such diabatic installations are therefore poor at satisfying the new European Community requirements concerning energy, economic, and environmental performance that require energy installations to be implemented that present high efficiency and that are environmentally friendly.
In order to mitigate those drawbacks, proposals have been made for installations that store energy by adiabatic compression of air (known as advanced adiabatic compressed air energy storage (AA-CAES)). That type of installation presents the feature of recovering the heat due to compression in reversible heat storage operating at high temperature. For this purpose, a heat regenerator forming part of the installation serves firstly to recover and store the heat energy generated by the compression—before the air is stored in the cavity—and secondly to restore this heat energy to the stream of expanding air in order to heat it before it is fed to the air turbine. Having recourse to a heat regenerator thus enables such installations to operate without CO2 emissions and enables their electrical efficiency to be raised to more than 70%.
In such installations, the regenerator has the particularity of being subjected in alternation to the same stream of air that flows in one direction and then in the other: the stream of compressed air that passes through the regenerator during compression stages while yielding its heat energy is the same as the stream that passes therethrough in order to be heated during expansion stages that occur in alternation with the compression stages. Under such conditions, the temperature of the air at the outlet from the regenerator (in a compression stage) is influenced by the inlet temperature of the air during an expansion stage. Unfortunately, if the regenerator is not regulated efficiently, then the temperature of the air at the outlet from the regenerator during compression will increase progressively over successive operating cycles of the installation because the amount of energy extracted from the regenerator during the expansion stages does not compensate the quantity of heat stored during compression stages, in spite of heat losses through the walls. This temperature hysteresis is unacceptable since air temperatures above a critical threshold (about 50° C.) at the inlet to the storage capacity cannot be tolerated. Exceeding this threshold requires the operation of the installation to be stopped immediately on pain of rapidly degrading the organic layer in the storage capacity (by heating it excessively), or even of causing the steel involved in its design to buckle (by losing mechanical strength as a result of going beyond the limit for plastic deformation). Furthermore, progressive and irreversible overheating of the regenerator makes it impossible to perform any kind of control of its outlet temperatures.
As a result, a heat regenerator used in an installation for mass energy storage by adiabatic compression of air must perform two thermal functions. Firstly it must enable substantially all of the heat generated by the compressors to be stored under good conditions of thermodynamic reversibility. Secondly it must enable the outlet temperature of the air to be controlled so as to occupy narrow ranges (during both compression and expansion stages) in order to guarantee that the equipment (such as the compressors, the air turbines, and the storage cavity) continues to operate and remains unharmed.